Electrostatic actuator apparatus

ABSTRACT

According to one embodiment, an electrostatic actuator apparatus includes a first voltage generation circuit configured to generate a first voltage, a first switch connected between the first voltage generation circuit and a first node, a second voltage generation circuit configured to generate a second voltage, a second switch connected between the second voltage generation circuit and a second node, a capacitor connected between the first node and the second node, an electrostatic actuator having a drive electrode connected to the first node, and a control circuit configured to perform an operation of sequentially turning on the first switch, turning off the first switch and turning on the second switch when the electrostatic actuator is driven.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 12/817,336filed Jun. 17, 2010; the entire contents of which are incorporatedherein by reference.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-031820, filed Feb. 16, 2010; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an electrostaticactuator apparatus.

BACKGROUND

Radio frequency (RF) micro-electromechanical systems (MEMS) variablecapacitors and RF-MEMS switches utilizing an MEMS technique aredeveloped and electrostatic actuators are used in the MEMS variablecapacitors and MEMS switches (for example, JP 2006-123110).

An actuation voltage of the electrostatic actuator is generated by useof a booster circuit provided in a semiconductor device (for example, JPH7-160215 and JP 2004-336904). In order to drive the electrostaticactuator, for example, it is required to use a high voltage of 10 V ormore. It takes a long time for the booster circuit to generate the highvoltage and, as a result, the switching speed of the MEMS is lowered.Further, the circuit area of the booster circuit that generates the highvoltage becomes large and the manufacturing cost will become high.

When the electrostatic actuator is driven by use of the high voltage,stiction due to charging increases and a fault tends to occur.Therefore, it is desired to generate a high voltage required for drivingthe electrostatic actuator in a short time and make it difficult tocause charging.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram showing the configuration of an electrostaticactuator apparatus 10 according to a first embodiment.

FIG. 2 is a schematic diagram showing the configuration of one switchunit 15 and one electrostatic actuator 16.

FIG. 3 is a cross-sectional view showing the structure of a variablecapacitor device 26.

FIG. 4 is a schematic diagram showing a plurality of electrostaticactuators 16.

FIG. 5 is a circuit diagram showing an example of the concreteconfiguration of the switch unit 15.

FIG. 6 is a circuit diagram showing another example of the configurationof the electrostatic actuator apparatus 10.

FIG. 7 is a circuit diagram showing an example of the configuration of alow-pass filter 31.

FIG. 8 is a schematic diagram showing the up state and down state of theelectrostatic actuator 16.

FIG. 9 is a timing chart for illustrating voltages applied to theelectrostatic actuator 16.

FIG. 10 is a circuit block diagram showing the configuration of a localbooster 30-1 shown in FIG. 5.

FIG. 11 is a circuit diagram showing an example of the configuration ofa booster circuit 32.

FIG. 12 is a block diagram showing the configuration of an electrostaticactuator apparatus 10 according to a second embodiment.

FIG. 13 is a schematic diagram showing the configuration of one switchunit 15 and one electrostatic actuator 16.

FIG. 14 is a block diagram showing the main portion of an electrostaticactuator apparatus 10 according to a third embodiment.

FIG. 15 is a block diagram showing the main portion of an electrostaticactuator apparatus 10 according to a fourth embodiment.

FIG. 16 is a graph showing the characteristics of a gate capacitor Cb.

FIG. 17 is a cross-sectional view showing the structure of a MOScapacitor Cb.

FIG. 18 is a graph showing the characteristics of the MOS capacitor Cb.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided anelectrostatic actuator apparatus comprising: a first voltage generationcircuit configured to generate a first voltage; a first switch connectedbetween the first voltage generation circuit and a first node; a secondvoltage generation circuit configured to generate a second voltage; asecond switch connected between the second voltage generation circuitand a second node; a capacitor connected between the first node and thesecond node; an electrostatic actuator having a drive electrodeconnected to the first node; and a control circuit configured to performan operation of sequentially turning on the first switch, turning offthe first switch and turning on the second switch when the electrostaticactuator is driven.

The embodiments will be described hereinafter with reference to theaccompanying drawings. In the description which follows, the same orfunctionally equivalent elements are denoted by the same referencenumerals, to thereby simplify the description.

First Embodiment

FIG. 1 is a block diagram showing the configuration of an electrostaticactuator apparatus 10 according to a first embodiment. The electrostaticactuator apparatus 10 comprises n (n is an integer greater than zero)electrostatic actuators 16, a drive circuit 11 that drives theelectrostatic actuators 16 and a control circuit 17 that controls theoperation of the drive circuit 11. The drive circuit 11 comprises anoscillator (OSC) 12, voltage generation circuit 13, limiter (LIM) 14 andn (equal to the number of electrostatic actuators 16) switch units 15.The drive circuit 11 and electrostatic actuators 16 are formed on thesame substrate, for example.

For example, the voltage generation circuit 13 is configured by abooster circuit and boosts power supply voltage Vdd to generate voltageVpp that is higher than power supply voltage Vdd. The booster circuit isconfigured by a charge pump, for example. The charge pump performs apump operation by using a clock CLK supplied from the oscillator 12.

The limiter 14 is connected to the voltage generation circuit 13. Thelimiter 14 prevents an output voltage of the voltage generation circuit13 from becoming higher than a preset voltage. Thus, the voltagegeneration circuit 13 can output a stable voltage.

FIG. 2 is a schematic diagram showing the configuration of one switchunit 15 and one electrostatic actuator 16.

For example, the electrostatic actuator 16 comprises a first electrode21 provided on an insulating substrate 20, an insulating film 22 formedon the first electrode 21 and a second electrode 23 that is providedabove the insulating film 22 and is vertically movable. The secondelectrode 23 is connected to a fixed portion 25 via an elastic body (forexample, a spring) 24. The insulating substrate 20 is configured by aninsulating layer formed on a silicon substrate or glass substrate. It isonly required for the insulating film 22 to play a role of preventingthe first electrode 21 and second electrode 23 from being made toelectrically contact each other. Therefore, it is sufficient to providethe insulating film 22 between the first electrode 21 and the secondelectrode 23. For example, the insulating film 22 may be provided onlyon the lower surface of the second electrode 23 or formed on both of theupper surface of the first electrode 21 and the lower surface of thesecond electrode 23.

The first electrode 21 is grounded, that is, ground voltage Vss isapplied to the first electrode 21. The second electrode 23 is connectedto the switch unit 15 via a node N1. Therefore, the second electrode 23can vertically move according to a voltage applied thereto.

The electrostatic actuator may configure a part of a variable capacitordevice 26 shown in FIG. 3, for example. A first electrostatic actuator16A is configured by a first electrode 21A, insulating film 22A, secondelectrode 23A, spring 24A and fixed portion 25A. A second electrostaticactuator 16B is configured by a first electrode 21B, insulating film22B, second electrode 23B, spring 24B and fixed portion 25B. A variablecapacitor 16C comprises a first electrode 21C formed on the substrate20, an insulating film 22C formed on the first electrode 21C and asecond electrode 23C that is provided above the insulating film 22 andis vertically movable.

Both sides of the second electrode 23C are fixed on the secondelectrodes 23A and 23B with insulating layers disposed therebetween andcan vertically move according to the movement of the second electrodes23A and 23B. Thus, the first electrode 21C, insulating film 22C andsecond electrode 23C may function as a variable capacitor. Theelectrostatic actuator can be applied to a device other than thevariable capacitor device and can be applied to a switch, for example.

The switch unit 15 comprises switches SW1 and SW2 and a capacitor Cb.One end of switch SW1 is connected to the output terminal of the voltagegeneration circuit 13 and the other end of switch SW1 is connected tothe first electrode of the capacitor Cb via node N1. One end of switchSW2 is connected to the output terminal of the voltage generationcircuit 13 and the other end of switch SW2 is connected to the secondelectrode of the capacitor Cb via a node N2. The switch unit 15 controlsthe operation of switches SW1 and SW2 to apply an actuation voltage tothe second electrode 23 of the electrostatic actuator 16.

The number of electrostatic actuators 16 can of course be set to aplural number. FIG. 4 is a schematic diagram showing a plurality ofelectrostatic actuators 16. In this case, the electrostatic actuatorapparatus 10 comprises a plurality of switch units 15-1 to 15-ncorresponding in number to a plurality of electrostatic actuators 16-1to 16-n. One switch unit 15 has a role of applying an actuation voltageonly to a corresponding one of the electrostatic actuators 16. Thevoltage generation circuit 13 is provided only one for a plurality ofelectrostatic actuators 16-1 to 16-n. In the following explanation, theconfiguration and operation of one electrostatic actuator 16 areexplained, but the configuration and operation of the otherelectrostatic actuators are the same as those described below.

FIG. 5 is a circuit diagram showing an example of the concreteconfiguration of the switch unit 15. As switches SW1 and SW2, metaloxide semiconductor field effect transistors (MOSFETs) for highwithstanding voltage are used and, for example, N-channel MOSFETs(NMOSFETs) are used.

The drain of the NMOSFET (SW1) is connected to the output terminal ofthe voltage generation circuit 13 and the source of the NMOSFET (SW1) isconnected to the first electrode of the capacitor Cb via node N1. Thegate voltage of the NMOSFET (SW1) is controlled by a local booster 30-1.

The drain of the NMOSFET (SW2) is connected to the output terminal ofthe voltage generation circuit 13 and the source of the NMOSFET (SW2) isconnected to the second electrode of the capacitor Cb via node N2. Thegate voltage of the NMOSFET (SW2) is controlled by a local booster 30-2.

Node N1 is connected to a first discharging circuit D1. The firstdischarging circuit D1 discharges node N1 based on a control signal CS1from the control circuit 17. Node N2 is connected to a seconddischarging circuit D2. The second discharging circuit D2 dischargesnode N2 based on a control signal CS2 from the control circuit 17. Eachof the first discharging circuit D1 and second discharging circuit D2 isconfigured by an N-channel MOSFET, for example.

A capacitor Cpara connected to node N1 represents a parasiticcapacitance associated with an interconnection and Cact represents thecapacitance of the electrostatic actuator 16. A combination of Cpara andCact is expressed by CL.

As shown in FIG. 6, a low-pass filter (LPF) 31 may be inserted betweenthe switch unit 15 and the electrostatic actuator 16. FIG. 7 is acircuit diagram showing an example of the configuration of the low-passfilter 31. The low-pass filter 31 comprises two resistors 31-1 and 31-2and a capacitor 31-3. Since noise and ripple from the voltage generationcircuit 13 can be reduced by using the low-pass filter 31, the operationof the electrostatic actuator 16 can be stabilized.

In this embodiment, nothing but the resistance element is connectedbetween the switch unit 15 and the electrostatic actuator 16, morespecifically, between the first electrode of the capacitor Cb and theelectrostatic actuator 16. This is a necessary condition to transmit anoutput voltage from the switch unit 15 to the electrostatic actuator 16as it is.

(Operation)

Next, the operation of the electrostatic actuator apparatus 10 with theabove configuration is explained. The electrostatic actuator 16 canselectively take an up state and down state according to a voltageapplied to the second electrode 23. When the output voltage of theswitch unit 15 becomes higher than or equal to an actuation voltage ofthe electrostatic actuator 16, the electrostatic actuator 16 transitsfrom the up state to the down state.

FIG. 8 is a schematic diagram showing the up state and down state of theelectrostatic actuator 16. The up state is a state in which theelectrostatic actuator 16 is not driven and the voltage of the secondelectrode 23 in the up state is not lower than ground voltage Vss and islower than a pull-in voltage. The pull-in voltage is a voltage requiredfor the second electrode 23 to be driven downwardly and is a voltage atwhich electrostatic attraction that attracts the second electrode 23towards the first electrode 21 becomes larger than the restoring forceof the spring 24.

In order to drive the electrostatic actuator 16, that is, in order tochange the state from the up state to the down state, switches SW1 andSW2 are operated in the following sequence. The operation of switchesSW1 and SW2 is controlled by the control circuit 17.

(1) SW1: On

(2) SW1: Off

(3) SW2: On

FIG. 9 is a timing chart for illustrating voltages applied to theelectrostatic actuator 16. In FIG. 9, voltages of nodes N1 and N2 inFIG. 5 are shown. It is supposed that an output voltage of the voltagegeneration circuit 13 is set to Vpp and capacitances of theelectrostatic actuator 16 in the up state and down state arerespectively set to Cact[up] and Cact[down].

If switch SW1 is turned on, voltage Vpp is applied to the electrostaticactuator 16. The voltage Vpp is set higher than or equal to a holdvoltage of the electrostatic actuator 16 and lower than a pull-involtage of the electrostatic actuator 16. Therefore, at this time point,the electrostatic actuator 16 is kept in the up state. The hold voltagerepresents the voltage required for maintaining a state set when thesecond electrode 23 is driven downwardly and made to contact theinsulating film 22. The hold voltage is lower than the pull-in voltage.

Next, after switch SW1 is turned off, switch SW2 is turned on. Ifvoltage Vpp is applied to node N2, the voltage of node N1 is boosted toactuation voltage Vact1 that is higher than the pull-in voltage andvoltage Vpp due to the capacitive coupling of the capacitor Cb.Actuation voltage Vact1 is given by the following equation.

Vact1={(2Cb+Cpara+Cact[up])/(Cb+Cpara+Cact[up])}·Vpp

If Vpp=22 V and Cb=CL (=Cpara+Cact), then Vact1=33 V. The electrostaticactuator 16 can be set in the down state by driving the electrostaticactuator 16 by use of voltage Vact1. According to the above system, theelectrostatic actuator 16 can be driven by the voltage generationcircuit 13 that generates voltage Vpp lower than voltage Vact1.Therefore, the area of the voltage generation circuit 13 can be reducedin comparison with a case wherein actuation voltage Vact1 is directlygenerated. Further, since it is only required for the voltage generationcircuit 13 to generate voltage Vpp lower than actuation voltage Vact1,the boosting time can also be reduced.

Further, according to this system, charging of the electrostaticactuator 16 can be suppressed. Charging represents a phenomenon wherecharges are accumulated on the insulating film 22 when the electrostaticactuator 16 repeatedly performs the drive operation and the secondelectrode 23 becomes difficult to separate from the insulating film 22due to the presence of charges. The influence of the charging phenomenonbecomes larger as the actuation voltage becomes higher. The reason whythe charging phenomenon can be suppressed is explained below.

The state of the electrostatic actuator 16 transits from the up state tothe down state when actuation voltage Vact1 is applied thereto, but thecharge amount of node N1 before and after the transition can bemaintained. At this time, the capacitance of the electrostatic actuator16 increases from Cact[up] to Cact[down]. As a result, the voltage ofnode N1 decreases from Vact1 to Vact2. Voltage Vact2 is set not lowerthan a hold voltage of the electrostatic actuator 16 and is set lowerthan voltage Vact1. Voltage Vact2 is given by the following equation.

Vact2={(Cb+Cpara+Cact[up])/Cb+Cpara+Cact[down]}·Vact1

As a result, an electric field applied to the insulating film 22 of theelectrostatic actuator 16 is reduced and the charging phenomenon issuppressed. When the electrostatic actuator 16 is restored to the upstate, nodes N1 and N2 are respectively discharged by the firstdischarging circuit D1 and second discharging circuit D2.

In order to realize the operation in FIG. 9, it is necessary to directlyapply the output voltage (actuation voltage) of the switch unit 15 tothe electrostatic actuator 16 without dropping the output voltage.Therefore, an element (for example, a switch or power source) thatchanges the output voltage of the switch unit 15 is not inserted betweenthe switch unit 15 and the electrostatic actuator 16. The low-passfilter 31 that includes a resistance element is a circuit that reducesvoltage noise and ripple. Even when the low-pass filter 31 is connectedbetween the switch unit 15 and the electrostatic actuator 16, the outputvoltage of the switch unit 15 can be directly applied to theelectrostatic actuator 16. In other words, an element that changes thevoltage and is different from the resistance element is not insertedbetween the switch unit 15 and the electrostatic actuator 16.

When switch SW2 is turned on and the voltage of node N1 is boosted toactuation voltage Vact1, a large potential difference is applied betweenthe gate and source of the NMOSFET configuring switch SW1. If the aboveoperation is repeatedly performed, the gate insulating film of theNMOSFET (SW1) is degraded. In order to prevent this, the gate voltage ofthe NMOSFET (SW1) is not set to zero, but to an intermediate voltagehigher than zero and lower than voltage Vpp when switch SW2 is turnedon. An example of the configuration of the local booster 30-1 thatrealizes the above operation is explained. FIG. 10 is a circuit blockdiagram showing the configuration of the local booster 30-1 shown inFIG. 5. The configuration of the local booster 30-2 is the same as thatshown in FIG. 10.

The local booster 30-1 includes a booster circuit 32, limiter (LIM) 33,first discharging circuit 34 and second discharging circuit 35. Thebooster circuit 32 generates a gate voltage that turns on the NMOSFETconfiguring switch SW1 by use of power supply voltage Vdd. The outputterminal of the booster circuit 32 is connected to switch SW1 via a nodeN3. The booster circuit 32 is configured by a charge pump, for example.The charge pump performs a pump operation by using a clock CLK suppliedfrom the oscillator 12.

The limiter 33 is connected to the booster circuit 32. The limiter 33prevents an output voltage of the booster circuit 32 from becominghigher than a preset voltage. Thus, the booster circuit 32 can output astable voltage.

The first discharging circuit 34 is connected to node N3. The firstdischarging circuit 34 is configured by an NMOSFET. The drain of theNMOSFET 34 is connected to node N3, the source of the NMOSFET 34 isgrounded and a control signal CS3 is supplied from the control circuit17 to the gate of the NMOSFET 34. The first discharging circuit 34discharges node N3 to approximately 0 V.

The second discharging circuit 35 is connected to node N3. The seconddischarging circuit 35 includes a diode group having a plurality ofdiodes 35A serially connected (cascade-connected) and an NMOSFET 35Bacting as a switching element. For example, each of the diodes 35A isconfigured by diode-connecting an NMOSFET. The anode of the first-stagediode 35A is connected to node N3.

The drain of the NMOSFET 35B is connected to the cathode of thefinal-stage diode 35A, the source of the NMOSFET 35B is grounded and acontrol signal CS4 is supplied from the control circuit 17 to the gateof the NMOSFET 35B. The second discharging circuit 35 discharges node N3to a voltage that is almost equal to a voltage corresponding to voltagedrops of the plural diodes 35A. Therefore, by setting the number ofdiodes 35A to an optimum value, the voltage of node N3 can be dischargedto a desired voltage higher than zero.

FIG. 11 is a circuit diagram showing an example of the configuration ofthe booster circuit 32. A clock CLK is supplied from the oscillator 12to the first input terminal of a NAND gate ND and an enable signal EN issupplied from the control circuit 17 to the second input terminal of theNAND gate ND. The output terminal of the NAND gate ND is connected tothe input terminal of an inverter INV1. The output terminal of inverterINV1 is connected to the input terminal of an inverter INV2.

Inverter INV1 outputs a clock CLK while the enable signal EN is high.Inverter INV2 outputs a clock bCLK obtained by inverting the clock CLKwhile the enable signal EN is high.

The first electrode of a capacitor 32A is connected to the anode of adiode 32B. The cathode of the diode 32B is connected to the anode of anext-stage diode. With this relation maintained, a plurality of unitseach configured by one capacitor 32A and one diode 32B are connectedwith the diodes 32B serially connected.

For example, the diode 32B is configured by diode-connecting an I(intrinsic)-type NMOSFET. Since the threshold voltage of the I-typeNMOSFET is approximately zero, the voltage drop of the diode 32B can besuppressed to approximately zero.

Voltage Vpp from the voltage generation circuit 13 is applied to theanode of the first-stage diode 32B. The cathode of the final-stage diode32B is connected to node N3. The plural capacitors 32A are alternatelysupplied with the clocks CLK and bCLK. As a result, the booster circuit32 can generate a voltage higher than voltage Vpp by transferringcharges stored on the capacitor 32A to the next-stage capacitor.

Next, the operation of the local booster 30-1 with the aboveconfiguration is explained. When switch SW1 is turned on, an on-voltageis applied to switch SW1 by the booster circuit 32. Then, when switchSW1 is turned off, node N3 is discharged by the first dischargingcircuit 34 of the local booster 30-1. As a result, the gate voltage ofthe NMOSFET of switch SW1 is set to approximately zero and switch SW1 isturned off.

Next, when switch SW2 is turned on, the NMOSFET 35B of the seconddischarging circuit 35 of the local booster 30-1 is turned on. At thistime, the first discharging circuit 34 of the local booster 30-1 is keptoff. As a result, node N3 is set to a voltage that is higher than groundvoltage Vss by the voltage drops of the plural diodes 35A, for example,approximately 10 V. For example, if Vact1=33 V, the potential differencebetween the gate and source of the NMOSFET of switch SW1 is set toapproximately 23 V and is set to a potential difference that is lower byapproximately 10 V than in a case where the gate voltage is set to zero.As a result, degradation in the gate insulating film of switch SW1 canbe suppressed.

(Effect)

As described above, in the first embodiment, the switch unit 15 isconnected between the voltage generation circuit 13 that generatesvoltage Vpp and the electrostatic actuator 16 and the switch unit 15generates actuation voltage Vact1 of the electrostatic actuator 16 thatis higher than voltage Vpp. The switch unit 15 comprises the capacitorCb, switch SW1 connected between the voltage generation circuit 13 andthe first electrode of the capacitor Cb, and switch SW2 connectedbetween the voltage generation circuit 13 and the second electrode ofthe capacitor Cb. When the electrostatic actuator 16 is driven, theoperation of sequentially turning on switch SW1, turning off switch SW1and turning on switch SW2 is performed.

Therefore, according to the first embodiment, actuation voltage Vact1higher than voltage Vpp generated by the voltage generation circuit 13can be applied to the electrostatic actuator 16 by the switch unit 15.As a result, the area of the voltage generation circuit 13 can bereduced and the manufacturing cost can be lowered.

Further, it becomes possible to generate actuation voltage Vact1 in anextremely short time. Thus, the switching speed of the electrostaticactuator 16 can be increased.

Further, when the electrostatic actuator 16 is driven, the actuationvoltage can automatically be lowered to voltage Vact2 that is lower thanvoltage Vact1 while the electrostatic actuator 16 is kept in the downstate after actuation voltage Vact1 is applied to the electrostaticactuator 16. As a result, the charging phenomenon of the electrostaticactuator 16 can be suppressed and a stiction fault can be reduced.

The potential difference between the gate and source of the NMOSFET ofswitch SW1 is reduced by the local booster 30-1 while switch SW1 is setoff and switch SW2 is set on. As a result, degradation in the gateinsulating film of the NMOSFET of switch SW1 can be prevented.

Second Embodiment

In the first embodiment, voltages applied to switches SW1 and SW2 areboth set to voltage Vpp generated by the voltage generation circuit 13,but the voltages are not limited to this case and a voltage applied toswitch SW1 and a voltage applied to switch SW2 may be different fromeach other. In the second embodiment, a drive circuit 11 comprises afirst voltage generation circuit 13-1 and second voltage generationcircuit 13-2 that respectively generate different voltages Vpp1 andVpp2. The first voltage generation circuit 13-1 applies voltage Vpp1 toswitch SW1 and the second voltage generation circuit 13-2 appliesvoltage Vpp2 to switch SW2.

FIG. 12 is a block diagram showing the configuration of an electrostaticactuator apparatus 10 according to the second embodiment. The drivecircuit 11 comprises the first voltage generation circuit 13-1 andsecond voltage generation circuit 13-2. The first voltage generationcircuit 13-1 is connected to a limiter 14-1 and the second voltagegeneration circuit 13-2 is connected to a limiter 14-2.

For example, the first voltage generation circuit 13-1 is configured bya booster circuit and boosts power supply voltage Vdd to generatevoltage Vpp1 higher than power supply voltage Vdd. The second voltagegeneration circuit 13-2 is configured by a booster circuit and boostspower supply voltage Vdd to generate voltage Vpp2 higher than powersupply voltage Vdd.

FIG. 13 is a schematic diagram showing the configuration of one switchunit 15 and one electrostatic actuator 16. Voltage Vpp1 from the firstvoltage generation circuit 13-1 is applied to one end of switch SW1.Voltage Vpp2 from the second voltage generation circuit 13-2 is appliedto one end of switch SW2. The other configuration is the same as thatshown in FIG. 2.

Voltage Vpp1 is set lower than a pull-in voltage of the electrostaticactuator 16 and is set not lower than a hold voltage of theelectrostatic actuator 16. Actuation voltage Vact1 is given by thefollowing equation.

Vact1=Vpp1+{Cb/(Cb+Cpara+Cact[up])}·Vpp2

The equation for voltage Vact2 is the same as that in the firstembodiment. Voltage Vpp2 is set to cause actuation voltage Vact1 tobecome not lower than the pull-in voltage of the electrostatic actuator16. Specifically, voltage Vpp2 is given by the following equation. Vpirepresents the pull-in voltage of the electrostatic actuator 16.

Vpp2≧{(Cb+Cpara+Cact[up])/Cb}·(Vpi−Vpp1)

Thus, even when different voltages are applied to switches SW1 and SW2while the voltage condition described above is satisfied, the sameoperation as that of the first embodiment can be realized. Therefore, inthe electrostatic actuator apparatus 10 according to the secondembodiment, the same effect as that of the first embodiment can beattained.

Third Embodiment

In the third embodiment, a first electrode 21 and second electrode 23 ofan electrostatic actuator 16 are respectively connected to switch units.The voltages of the first electrode 21 and second electrode 23 arecontrolled by the two switch units.

FIG. 14 is a block diagram showing the main portion of the electrostaticactuator apparatus 10 according to the third embodiment.

The output terminal of a voltage generation circuit 13 is connected toswitch units 15A and 15B. The output terminal of the switch unit 15A isconnected to the second electrode 23 of the electrostatic actuator 16via a low-pass filter (LPF) 31A. The output terminal of the switch unit15B is connected to the first electrode 21 of the electrostatic actuator16 via a low-pass filter (LPF) 31B. The configuration of the switchunits 15A and 15B is the same as that of the switch unit 15 shown inFIG. 5.

Next, the operation of the electrostatic actuator apparatus 10configured as described above is explained. When voltages Vact1 andVact2 are applied to the second electrode 23 of the electrostaticactuator 16, the switch unit 15A performs the operation explained in thefirst embodiment. At this time, the switch unit 15B sets the voltage ofthe first electrode 21 of the electrostatic actuator 16 to approximatelyzero. Specifically, the voltage of the first electrode 21 of theelectrostatic actuator 16 is set to approximately zero by turning offswitches SW1 and SW2 and discharging nodes N1 and N2 by dischargingcircuits D1 and D2.

Further, when actuation voltages Vact1 and Vact2 are applied to thefirst electrode 21 of the electrostatic actuator 16, the switch unit 15Bperforms the operation explained in the first embodiment. At this time,the switch unit 15A sets the voltage of the second electrode 23 of theelectrostatic actuator 16 to approximately zero.

Therefore, in the third embodiment, the direction of an electric fieldbetween the first electrode 21 and the second electrode 23 can bereversed while the operation of transiting the state of theelectrostatic actuator 16 from the up state to the down state is beingperformed. For example, if an electric field set only in one directionis continuously applied, charges are stored on an insulating film 22 ofthe electrostatic actuator 16 and a problem of charging occurs. However,in the third embodiment, for example, charges stored on the insulatingfilm 22 can be extracted by changing the direction of an electric fieldbetween the first electrode 21 and the second electrode 23 for eachpreset time. As a result, a charging phenomenon can be suppressed. Theother effects are the same as those of the first embodiment.

Further, the third embodiment can be applied to the second embodiment.

Fourth Embodiment

In the fourth embodiment, a capacitor element whose capacitance variesdepending on the potential difference between nodes N1 and N2 is usedinstead of the capacitor Cb in FIG. 2. Specifically, a capacitor elementhaving a characteristic that the capacitance thereof is reduced when thepotential difference between nodes N1 and N2 becomes smaller can beused. As the capacitor element having the above characteristic, (1) agate capacitor using an NMOSFET having a positive threshold value and(2) a MOS capacitor and the like can be provided.

(Gate Capacitor)

FIG. 15 is a block diagram showing the main portion of the electrostaticactuator apparatus 10 according to the fourth embodiment. Theelectrostatic actuator apparatus 10 is the same as the electrostaticactuator apparatus 10 of the first embodiment except that the capacitorCb in FIG. 2 is replaced by a gate capacitor.

The gate capacitor Cb is configured by connecting the drain and sourceof an NMOSFET having a positive threshold voltage. The gate of the gatecapacitor Cb is connected to node N1 and the drain and source of thegate capacitor Cb are connected to node N2.

FIG. 16 is a graph showing the characteristic of the gate capacitor Cb.In FIG. 16, the abscissa represents gate voltage Vg in volts and theordinate represents capacitance Cb in femtofarads. In FIG. 16, thecharacteristics of two types of gate capacitors having differentthreshold voltages are shown.

As shown in FIG. 16, the capacitance of the gate capacitor Cb becomessmaller when the gate voltage lies between a flat-band voltage and thethreshold voltage. Therefore, when the gate voltage of the gatecapacitor Cb (that is a voltage of node N1) becomes lower than thethreshold voltage, the capacitance of the gate capacitor Cb becomessmall. As a result, the voltage of node N1, that is, actuation voltageVact of the electrostatic actuator can be increased.

Therefore, according to the fourth embodiment, since voltage Vpp of thevoltage generation circuit 13 can be set low, the size of the voltagegeneration circuit 13 can be reduced. Further, the power consumption ofthe electrostatic actuator apparatus 10 can be reduced.

(MOS Capacitor)

A MOS capacitor can be used as the capacitor Cb. FIG. 17 is across-sectional view showing the structure of the MOS capacitor Cb.

An N-type well (N-well) 40 is formed in a P-type semiconductor substrate(not shown). In the N-type well 40, impurity regions 41 and 42 areseparately formed. The impurity regions 41 and 42 have the sameconductivity type as that of the N-type well 40 and are formed by dopingN⁺-type impurity with high concentration into the N-type well 40. A gateinsulating film 43 is formed on a portion of the N-type well 40 thatlies between the impurity regions 41 and 42 and a gate electrode 44 isformed on the gate insulating film 43. For example, the gate electrode44 is formed of polysilicon having N-type impurity doped therein.

A terminal T1 connected to the gate electrode 44 is connected to node N1and a terminal T2 connected to the impurity regions 41 and 42 isconnected to node N2.

The capacitance of the MOS capacitor Cb varies by controlling theimpurity concentration of the N-type well 40. FIG. 18 is a graph showingthe characteristic of the MOS capacitor Cb. In FIG. 18, the abscissarepresents gate voltage Vg in volts and the ordinate representscapacitance Cb in femtofarads.

As shown in FIG. 18, the capacitance of the MOS capacitor Cb can bevaried by changing the impurity implantation condition. Specifically, avariation amount of the capacitance of the MOS capacitor Cb can be madelarger as the impurity concentration becomes lower. The same effect asthat obtained when the gate capacitor is used can be attained when theMOS capacitor Cb is used.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. An electrostatic actuator apparatus comprising: a voltage generationcircuit configured to generate a first voltage; a first switch connectedbetween the voltage generation circuit and a first node; a second switchconnected between the voltage generation circuit and a second node; acapacitor connected between the first node and the second node; anelectrostatic actuator having a drive electrode connected to the firstnode; and a control circuit configured to perform an operation ofsequentially turning on the first switch, turning off the first switchand turning on the second switch when the electrostatic actuator isdriven.
 2. The apparatus of claim 1, wherein an element other than aresistor element is not connected between the capacitor and theelectrostatic actuator.
 3. The apparatus of claim 1, wherein the firstvoltage is lower than an actuation voltage of the electrostaticactuator.
 4. The apparatus of claim 1, wherein the first switchcomprises a first MOSFET and a first local booster that controls a gatevoltage of the first MOSFET, and the second switch comprises a secondMOSFET and a second local booster that controls a gate voltage of thesecond MOSFET.
 5. The apparatus of claim 4, wherein the first localbooster sets the gate voltage of the first MOSFET to a second voltagehigher than a ground voltage when the first MOSFET is turned off and thesecond MOSFET is turned on.
 6. The apparatus of claim 5, wherein thefirst local booster comprises a discharging circuit comprising a diodegroup and a third MOSFET of N-type, the diode group having diodesserially connected, one end of the diode group is connected to a gate ofthe first MOSFET, a drain of the third MOSFET is connected to the otherend of the diode group, and a source of the third MOSFET is grounded. 7.The apparatus of claim 4, wherein the first local booster furthercomprises a booster circuit that generates an on-voltage to turn on thefirst MOSFET.
 8. The apparatus of claim 1, further comprising: a firstdischarging circuit connected to the first node; and a seconddischarging circuit connected to the second node.
 9. The apparatus ofclaim 8, wherein the first discharging circuit comprises a MOSFET ofN-type whose drain is connected to the first node and whose source isgrounded, and the second discharging circuit comprises a MOSFET ofN-type whose drain is connected to the second node and whose source isgrounded.
 10. The apparatus of claim 1, further comprising a low-passfilter connected between the capacitor and the electrostatic actuator.11. The apparatus of claim 1, wherein the capacitor has a characteristicthat a capacitance of the capacitor becomes smaller as the potentialdifference becomes smaller.
 12. The apparatus of claim 11, wherein thecapacitor is a gate capacitor.
 13. The apparatus of claim 11, whereinthe capacitor is a MOS capacitor.
 14. The apparatus of claim 1, whereinthe voltage generation circuit is a booster circuit.
 15. The apparatusof claim 1, wherein the electrostatic actuator comprises a firstelectrode provided on an insulating substrate, a second electrode thatis provided above the first electrode and is vertically movable, aninsulating film provided between the first electrode and the secondelectrode, and an elastic body configured to support the secondelectrode, and the first electrode or the second electrode is connectedto the first node.